Low HDL levels are a major atherogenic risk factor for which no specific treatment is yet available. The plasma liquid transfer proteins, cholestryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), play a an important part in HDL homeostasis. CETP transfers HDL CE to triglyceride-rich lipoproteins which are then taken up in the liver, in a process of reverse cholesterol or endogenous hypercholesterolemia. We have used natural flanking region CETP transgenic (Tg) mice, and reporter gene constructs, to localize the CETP response to a novel element in the gene proximal promoter, and we have shown that this positive response is mediated in vitro by the nuclear receptor, LXR. Since LXRalpha is known to mediate sterol up-regulation of cyp7a, the rate- limiting enzyme in the conversion of cholesterol into bile salts, our findings suggest that LXRs may help to coordinate the response of a variety of genes involved in cholesterol removal from the body. In Aim 1 we will use LXRalpha and beta knock-out mice to evaluate the role of LXRs in lipid transfer protein gene regulation in vivo and to explore the overall role of these transcription factors in regulating genes involved in reverse cholesterol transport; these mice will also be crossed onto atherosclerosis-susceptible backgrounds in order to evaluate their role in atherogenesis. Aim 2 we will use recently developed PLTP knockout-out (PLTP0) mice to explore the role of PLTP in lipoprotein metabolism and atherogenesis. We will investigate possible redundancy of CETP and PLTP activities by crossing CETP into the LTP0 background. PLTP0 mice will also be crossed with apoB Tg mice and atherosclerosis studies performed. These studies will evaluated the hypothesis that phospholipid/apoA-IV/apoE vesicles accumulating in PLTP0 mice on a high fat diet may have anti-atherogenic PLTP in adding phospholipids to nascent HDL. This project is likely to provide definitive information on the molecular mechanism of lipid transfer protein regulation by sterols. It will also test the novel hypothesis that LXRs coordinate a program of reverse cholesterol transport with anti-atherogenic consequences. Finally, it will evaluate a new idea concerning PLTP deficiency as an anti- atherogenic state. This project will have strong interactions with Project 4, investigating mechanisms of HDL-mediated cholesterol efflux from macrophages, and will provide information on novel genes and pathways regulated by LXRs, which may be relevant to studies in Projects 1 and 2.